1. Field of the Invention
The present invention relates to oxadiazole derivatives. In addition, the present invention relates to light-emitting element materials, light-emitting elements, and electronic devices each using the oxadiazole derivative.
2. Description of the Related Art
An organic compound can take a wider variety of structures compared with an inorganic compound, and it is possible to synthesize a material having various functions by appropriate molecular design of an organic compound. Owing to those advantages, electronics utilizing a functional organic material has been attracting attention in recent years.
For example, a solar cell, a light-emitting element, an organic transistor, and the like are exemplified as electronic devices utilizing an organic compound as a functional material. These are devices taking advantage of electric properties and optical properties of the organic compound. Among them, in particular, a light-emitting element has been making remarkable development.
It is considered that light emission mechanism of a light-emitting element is as follows: when a voltage is applied between a pair of electrodes between which a light-emitting layer is interposed, electrons injected from a cathode and holes injected from an anode are recombined in the light-emission center of the light-emitting layer to form a molecular exciton, and energy is released to emit light when the molecular exciton relaxes to a ground state. Singlet excitation and triplet excitation are known as excited states, and light emission can probably be achieved through either of the excited states.
Such a light-emitting element has a lot of problems which depend on the organic materials. In order to solve these problems, improvement of an element structure, development of a material, and the like have been carried out.
As the most basic structure of a light-emitting element, the following structure is known: a hole-transporting layer including an organic compound having a hole-transporting property and an electron-transporting light-emitting layer including an organic compound having an electron-transporting property are stacked to form a thin film of approximately 100 nm thickness in total, and this thin film is interposed between electrodes (see Non-Patent Document 1, for example).
When a voltage is applied to the light-emitting element described in Non-Patent Document 1, light emission can be obtained from the organic compound having a light-emitting property and an electron-transporting property.
Further, in the light-emitting element described in Non-Patent Document 1, functions are separated as appropriate so that the hole-transporting layer transports holes whereas the electron-transporting layer transports electrons and emits light. However, various interactions (e.g., exciplex formation) frequently occur at an interface of the stacked layers. As a result, changes in the emission spectrum or reduction in emission efficiency may occur.
In order to suppress the changes in the emission spectrum or the reduction in emission efficiency, which are caused by the interactions at the interface, a light-emitting element having further functional separation has been developed. For example, a light-emitting element has been proposed, in which a light-emitting layer is interposed between a hole-transporting layer and an electron-transporting layer (see Non-Patent Document 2, for example).
In such a light-emitting element described in Non-Patent Document 2, in order to more effectively suppress the interaction occurring at the interface, it is preferable that the light-emitting layer be formed to include a bipolar organic compound having both an electron-transporting property and a hole-transporting property.
However, most organic compounds are monopolar materials having either a hole-transporting property or an electron-transporting property.
Therefore, bipolar organic compounds having both an electron-transporting property and a hole-transporting property should be developed.